Complex cell-cell interactions in tissues are the basis for their normal and pathological behavior. HIV infection disrupts multiple cell interactions in the lymphoid tissue where critical events of HIV disease occur. In vivo HIV-1 infection is transmitted by viruses that in vitro recognize chemokine receptor CCR5 (R5 variants), while viruses dominating later stages of HIV disease often recognize either CXCR4 alone (X4 variants) or in addition to CCCR5 (R5X4 variants). This switch of viral tropism often coincides with dramatic loss of CD4+ T cells. In ex vivo tissues R5 infection leads to severe depletion of CD4+ T cells, whereas R5 infection depletes CD4+ T cells only mildly. It is not known why R5 HIV-1 infection ex vivo does not lead to a massive CD4+ T cell depletion, as it occurs in X4 HIV-infection. To answer this question we determined whether viral coreceptor preferences dictate the selective depletion of T- cells expressing particular coreceptors. We measured depletion of coreceptor-specific CD4+ T-cell subsets in histocultures inoculated with sets of X4 and R5 HIV-1 variants. Flow cytometry of cells isolated from cultured uninfected control tissues revealed that the CD4+ T lymphocyte population comprised subsets of cells differentially expressing CCR5 and CXCR4 HIV-1 coreceptors on their surface. CD4+ T lymphocytes were unequally distributed between CCR5+ and CXCR4+ cell subtypes; about 80% of CD4+ T lymphocytes were exclusively CXCR4+ whereas 4% of CD4+ T lymphocytes were exclusively CCR5+. 6% of CD4+ T cells expressed both CXCR4 and CCR5, and in the remaining CD4+ T cells neither CCR5 nor CXCR4 was detected on the cell surface. Infection with the X4 variants LAV.04 and NL4-3 or with a primary isolate 92US076 caused severe depletion of total CD4+ T cells, only 10 to 20% of CD4+ T cells remained in the infected lymphoid tissues on days 12 to 13 post-infection. In contrast, infection with all the tested R5 variants (laboratory strains BaL, SF162 and with a primary isolate 91US056) caused only mild depletion: about 90% of CD4+ T cells remained in the tissues compared to matched uninfected controls. However, when individual subsets of CD4+ T cells were examined, we found that R5 variants selectively depleted CCR5+/CD4+ T cells. No CCR5-/CD4+ T lymphocytes were significantly depleted by either of R5 HIV-1 variants tested. The depletion of CCR5+/CD4+ T lymphocytes accounts for the entire loss of CD4+ T cells in R5 HIV-1-infected lymphoid tissues. In contrast to R5 HIV-1, all three tested X4 variants severely depleted all subsets of CD4+ T cells, although these viruses depleted CXCR4+/CD4+ T cells slightly more efficiently than lymphocytes from other subsets. These data indicate that both X4 and R5 HIV-1 variants are highly cytopathic for their cognate CD4+ T cell targets in human lymphoid tissue. To test whether HIV variants that are classified as dual-tropic based on their usage of co- receptors in transfected cell lines, behave as dual-tropic in ex vivo lymphoid tissues, we used cloned R5X4 isolate 89.6 and its two R5X4 chimeras 89-v345.SF and 89-v345.FL which are isogenic except for specific env sequences. We treated tissues with specific coreceptor ligands, RANTES to inhibit viral entry through CCR5, and AMD3100 to block viral entry through CXCR4. R5X4 HIV-1 isolate 89.6 was almost completely inhibited by AMD3100, but was minimally affected by RANTES. Infection of 89-v345.SF was almost completely blocked by RANTES and was slightly inhibited by AMD3100. Infection with 89-v345.FL was almost equally sensitive to AMD3100 and RANTES. Thus, in the complex microenvironment of human lymphoid tissue some R5X4 variants behave as more X4, while others are more R5. Preferential usage of CXCR4 by these variants correlated with their cytopathicity: 20% of CD4+ T cells remained in 89.6-, 85% in 89- v345.SF-, and 50% in 89-v345.FL-infected tissues compared with uninfected controls. Similar to the pattern of cytopathicity of monotropic HIV variants, 89-v345. SF depleted only CCR5+/CD4+ T cells whereas, 89.6 and 89-v345. FL depleted all CD4+ T cell subsets. In summary, dual-tropic viruses may use only one co- receptor in lymphoid tissue, and for both mono- and dual-tropic HIV- 1 variants their cytopathicity toward the general CD4+ T cell population in lymphoid tissue is closely associated with use of CXCR 4. Apparent low cytopathicity of variantspreferentially using CC R5 in human lymphoid tissue is explained by the low frequency of CCR5 expressing cells.